Solar or photovoltaic cells (PVCs) are semiconductor devices having P-N junctions which directly convert radiant energy of sunlight into electrical energy. Conversion of sunlight into electrical energy involves three major processes: absorption of sunlight into the semiconductor material; generation and separation of positive and negative charges creating a voltage in the PVC; and collection and transfer of the electrical charges through terminal connected to the semiconductor material. PVCs are widely known and commonly used in a variety applications, including providing electrical energy for satellites and other space craft, marine vessels, installations in areas not served by a grid of an electric utility company, and portable consumer electronics devices such as radios, tape/compact disc players and calculators.
Heretofore PVCs have not been widely used as a main or even auxiliary source of power for residences and businesses having access to conventional power sources, for example, through a power grid of an electric utility company. There are several reasons for this, the most important of which is cost. Electricity produced from solar cells tends to be relatively expensive compared to that available from conventional power sources such as hydroelectric, oil-fired, coal fired and nuclear power plants.
Although the cost of installing, maintaining and repairing solar electric generation arrays or systems is not insignificant, the greatest cost associated with solar energy is the cost of the manufacturing the PVCs. Referring to FIG. 1, prior art PVCs 20 are typically formed on an ultra-pure silicon wafer or substrate 22, which in itself can cost from about 300 hundred to about 5 thousand dollars apiece depending on size. For example, an 8 inch diameter silicon commonly used in manufacturing PVCs typically costs about 2.5 thousand dollars. Furthermore, traditionally a large number of individual PVCs 20 were fabricated on a single substrate 22 by (i) depositing or growing a doped layer of semiconductor material, such as silicon, over the substrate 22 including a dopant of an opposite type; (ii) patterning and etching the substrate 22 with the doped layer thereon to form individual PVCs 20; (iii) depositing a metal layer over the etched substrate 22; (iv) patterning and etching the metal layer to form vias, contacts and lines interconnecting the individual PVCs 20; and (v) inspecting and testing the finished PVCs 20 to remove from an output circuit defective PVCs. The time, equipment and skilled operators required to perform each of the above steps makes the cost of solar electricity extremely expensive, and impractical for just about any use for which an alternative conventional energy source is available.
In an effort to reduce costs, some of the latest generations of PVCs have been monolithic PVCs in which substantially the entire surface of a substrate is taken up by a single large PVC, thereby eliminating much of the time and costs associated with patterning and etching the doped layer and the metal layer. However, this approach has not been wholly successful, since unlike with a substrate having numerous individual PVCs which can be individually removed from the output circuit, a single defect at any point in the monolithic PVC would render the entire substrate useless. In practice, this has resulted in yields well below 40%, offsetting or completely negating any cost savings realized with this approach.
Yet another problem with prior art PVCs is their low external quantum efficiency. By external quantum efficiency it is meant the proportion of the available photons converted into electrical energy. Power from the sun arrives at Earth in the form of photons of light in a wide spectrum from approximately 120 nanometers to 20 micrometers. The total solar irradiance, neglecting absorption in the atmosphere, is approximately 135 mW/cm2 (about 10,000 watts per square meter). Thus, a significant amount of solar radiation is available, but is not absorbed by today's commercially available PVCs. The challenge to photovoltaic manufacturers has always been how to convert this abundance of energy into electricity.
Inefficiency in converting available light into electrical energy is particularly a problem for solar electric systems having limited power generating capability. This is because usable solar energy is available for only a fraction of a day, when it is available the PVCs must generate energy to meet current demands and generate sufficient energy to be stored for use when usable solar energy is unavailable. Thus, conventional solar electric systems must either have relatively large numbers of PVCs, which as explained above are costly, or have a high degree of efficiency. Unfortunately, prior art PVCs are typically only from about 10 to 14% efficient.
Referring to FIG. 2 it is seen that a major reason for this poor efficiency comes from the reflectance of photons from front and buried surfaces of the PVCs. External Quantum Efficiency is reduced by the reflected photons, which either never enter the cell (front surface reflection) or are reflected from the back surface or metallization layer interfaces and exit the cell without being absorbed. Thus a significant or even a large proportion of the light incident on a surface 24 of the PVC 20 is simply reflected away again.
A more fundamental problem is due to quantum mechanical properties of the semiconductor crystal of the PVCs. Conventional PVCs are capable of utilizing or converting into electricity only a narrow range of light wavelengths corresponding to a band-gap energy of the p-n junction of the PVC, no matter how much light is concentrated or incident thereon. For example, although solar radiation includes wavelengths from 2×10−7 to 4×10−6 meters, silicon based PVCs having a band gap energy of about 1.1 electron volts (eV) are capable of utilizing only wavelengths from about 0.3×10−6 to about 3.0×10−6 meters. Similarly, gallium-arsenide (GaAs) based PVCs, aluminum-gallium-arsenide (AlGaAs) based PVCs, and germanium (Ge) based PVCs have band gap energies of 1.43, 1.7 and 0.5 eV respectively, and are therefore sensitive to other wavelengths.
Accordingly, there is a need for a solar collector that is inexpensive to fabricate, highly efficient in its utilization of available solar radiation, and which has an extended operational life.
The present invention provides a solution to these and other problems, and offers other advantages over the prior art.